The head of the United Nations nuclear watchdog has arrived in Tehran for talks on October 29 with senior Iranian officials, as opposition from the United States threatens to undermine a landmark international agreement to limit Iran's nuclear program.

Yukiya Amano, the director general of the International Atomic Energy Agency (IAEA), "will focus on the IAEA's verification and monitoring of Iran's implementation" of the 2015 accord, the IAEA said October 25.

The visit comes amid a dispute between Washington and Tehran over U.S. President Donald Trump's decision this month not to certify Iranian compliance with the nuclear deal between Iran and six world powers.

Trump, in a long-awaited Iran-policy speech on October 13, assailed Tehran as a "rogue regime" and threatened to walk away from the deal if what he called "serious flaws" are not fixed.

He slammed Tehran for what he said are violations of the “spirit” of the agreement, in part for its continued testing of ballistic missiles and its support for extremists in the Middle East.

The U.S. Congress now has fewer than 60 days to decide whether to reimpose economic sanctions on Tehran that were lifted under the deal.

In the 2015 deal, signed during the presidency of Barack Obama, Tehran agreed to curtail its nuclear activities in exchange for relief from international sanctions. Other signatories are Russia, China, Britain, France, and Germany.

Many U.S. and other officials see the deal as a way to slow Iran's potential path toward creating a nuclear weapon. Tehran says its nuclear program has purely peaceful aims.

Iranian state news agencies reported that Amano will meet with Vice President Ali Akbar Salehi, who oversees Iran's nuclear activities, among others.

Amano, whose agency is in charge of monitoring the deal's restrictions, said the accord subjects Tehran to the world's toughest nuclear inspection regime.

He said his inspectors have so far encountered no problems in investigating and determining whether Iran is complying with the deal.

European Union leaders have reaffirmed their commitment to the accord.

Morteza Saffari Natanzi, a member of the National Security and Foreign Policy Committee in the Majlis, Iran’s parliament, on October 28 urged European participants to help protect the accord.

He added that Tehran has made it clear to European countries that the deal was not open to renegotiation.

Iranian Supreme Leader Ayatollah Ali Khamenei has said Tehran will stick to the agreement as long as the other signatories do, but will "shred" the deal if Washington pulls out, as Trump has threatened to do.